Tapu Bulu introducted in gen7 has been one of the most used pokemons by far and that is no wonder when you see that it has great stats, good typings and a large movepool. It has been paired with Heatran for those reasons, Heatran is immune or resists all Tapu Bulu's weaknesses and Tapu Bulu resists all of Heatran's weaknesses which makes them a good combo. Finally, the core is missing several key components, one is speed, two, reliable momentum and power, three, checks to common sets that carry coverage against both both bulu and heatran for example Landorus-T and FlyiniumZ , Flying/Fighting like tomohawk or pinsir. Celesteela is there to prevent those threats from sweeping your team. Mega-Metagroos has a great amount of bulk as well as speed and attack. Pinsir is completely walled by it and so is Fidgit or Crucibelle-Mega . It also walls lele which is a big threats to any team. Tapu Bulu and Heatran can really beneficiate from Mega-Metagross as well as Mega Metagross can benefit from fire immunity and Grassy terrain.
SETS: Tapu bulu is banded, woodhammer is used to hit really hard and pressure your oponent, stone edge and superpower are your coverage and hornleech is to stay healthy. You also have 30 ivs in speed, that is for mega scizor to let it u-turn before you attack it so that you're able to hit hard whatever comes in next. Heatran is your stall breaker, it has taunt and toxic for stall, earthpower for fire types and other heatrans, magma storm to trap threats such as magearna, magnezone, tangrowth, ect. Mega-Metagross can pressure the opponent especially since it walls most of the common weaknesses of Heatran and Bulu, therefore the set is purely offensive (to keep the core balanced as well) It has Bullet Punch for priority, Meteordash which 2ohko Flyinium Z Landoru-t as well as hitting super hard and eventually get an attack boost, it also has earthquake as coverage for anything that can be a threat such as Crucibelle or Fidgit and finally Zen Headbutt as stab and for the rare fighting types such as Tomohawk that may come across.
TEAMMATES: I have chosen those teammates for the only reason that they can get pass what the core doesn't. They would work great in a team.
Landorus-t: is great as physical switch in and general wall +hazards +ground immunity
Mantine: pretty much the same but brings defog instead of stealth rocks +water and ground immunity
Scarf Tapu lele: is a very good threat, adds speed and boosts Mega-Metagross
Tapu koko: great cleaner and pivot boosts Mega Metagross and Second check to Pinsir